1. Field of the Invention
The present invention relates generally to optical diffusers including diffusive wire-grid polarizers.
2. Related Art
Wire-grid polarizers have been developed that are capable of polarizing light, i.e. separating one polarization orientation from another, by transmitting one polarization orientation and reflecting the other. Wire grid polarizers are a periodic structure of conductive elements with a length greater than the wavelength and a period (p) less than half the wavelength of the incident light, or p≦λ/2. Wire grid polarizers have been proven to be effective for visible light (˜300-700 nm, or ˜0.3-0.7 microns or μm) and their use demonstrated as polarizers and beam splitters in optical imaging systems. Typically, however, the reflection from, and the light passing through, such wire-grid polarizers, is specular, or mirror-like.
Wire-Grid polarizers are different from diffraction gratings, which are a periodic structure of dielectric material with a period (p) greater than half the wavelength (λ) of incident light, or p≧λ/2. The diffraction grating scatters the incident light at discrete angles or directions in accordance with mλ=p sin ⊖, where m is the order and ⊖ is the angle with respect to normal from the diffraction grating. Thus, different wavelengths are reflected or scattered at different angles.
Various different types of wire-grid polarizers have been proposed that include patterning the wires incurved lines, rather than strait lines; or forming the wires in a lattice structure with reinforcing members. See US Patent Application Publication US 2002/0167727 A2; and U.S. Pat. Nos. 6,972,906; 7,009,768; and PCT Application PCT/US2005/032656 (WO 2006/036546).
Other types of wire-grid polarizers have been proposed to diffusely reflect incident light that include contoured surfaces at different angles. See U.S. Pat. Nos. 6,081,376 and 6,348,995. Such polarizers, however, still specularly reflect, only from within several differently oriented textured surfaces.
Sometimes it is desirable to reflect all incident light or transmit most or all incident light in a diffuse manner. In this situation a diffuser is desired, but not a polarizing diffuser. One example of this situation would be a thermal window with a metallic film. Without a diffuser, a specular reflection would result from the building windows. Another need for diffuse light may be in an LCD display or a projector system.